I think I have the same problem, and probably the same 'cheapy PS2 style USB vibration gamepad'.
I'm using LOMAC (1st one) it's been in the cupboard collecting dust for years, but i thought I'd try it on a laptop, with the aforementioned cheapy PS2 style gamepad. Using keyboard to fly, ie without gamepad plugged in, I can fly and use pitch trim (ctrl-. and ctrl-; ) but as soon as I try the gamepad, no matter how I assign buttons and axes, the trim will not work, and even keyboard trim disappears.
I was just about at the end of my tether with it, when I thought I'd disconnect the gamepad and try flying with mouse and keyboard. That led to the workaround for this problem. With using mouse for normal flight controls there is a problem that it doesn't self centre. So I thought, what about using one of the mouse axes as the pitch trim control. I plugged the gamepad back in and set up the mouse scroll wheel (axis 2 on my mouse) as the pitch control. It works!! woohoo! Now to set the gamepad stick sensitivity, I use almost linear response, set curve slider just above minimum, and set shift slider quite high so that the response curve is quite shallow. I've done this for pitch and roll, gamepad axes 1 and 0 respectively, didnt bother with rudder which is on axis 3 (rotator ?).
At last I can fly and spend some time doing something else other than constantly trying to keep level, which is what was happening without a pitch trim control.
